Technical Field
The present invention relates generally to the field of fasteners. More particularly, the present invention relates to threaded nuts. Specifically, the present invention relates to thin walled fastener nuts used in the aerospace and other related industries.
Background Information
Conventional lightweight hex-flange thin walled nuts are usually made from steel and have a very thin flat wall for receiving a wrench or socket. These conventional thin walled nuts include a wide range of available torque values and high tensile heat treat values.
Some thin walled nuts are susceptible to failure due to cracking. Cracking in a nut may occur when the nut is too brittle. One way in which the brittleness of metal increases is related to the high heat treatment the nuts go through to harden the nuts. This is sometimes referred to as hydrogen embrittlement.
In aerospace applications, some fasteners must meet the minimum requirements set forth in the National Aerospace Standard Specification MS21042 (NASM21042). However, note 3 contained in NASM21042 also states that self-locking nut fasteners must be used within the limitations provided in National Aerospace Standard MS33588 (NASM33588). Other fasteners must meet the minimum requirements set forth in the National Aerospace Standard Specification NAS1291 (NAS1291).
The limitations of NASM33588 state that self-locking nuts shall not be used at joints in control systems at single attachments, or where loss of the bolt would affect safety of flight unless the threaded parts are held by a positive locking device that requires shearing or rupture of materials before torsional loads would relieve the initial stress of the assembly. Further, self-locking nuts shall not be used on any externally threaded part that serves as an axis of rotation for another part unless there are no possible torsional loads which can be applied to either the externally or internally threaded part in such a manner as to relieve the initial stresses of the assembly, or unless the threaded parts are held by a positive locking device that requires shearing or rupture of the material before torsional loads would relieve the initial stresses of the assembly.
With continued reference to NASM33588, self-locking nuts shall not be used with bolts or screws on jet engine aircraft in locations where a loose nut, bolt, or screw could fall or be drawn into the engine air intake duct. Self-locking nuts shall not be used with bolts, screws, or studs to attach panels, doors or to assemble any parts that are routinely disassembled prior to or after each florid for access or servicing.
Some exemplary nuts satisfying NASM21042 and NAS1291 sometimes still fail due to cracking. Some other failures have been due to design, application and installation. The original purpose of the exemplary nuts was to reduce weight and they were never intended for critical applications. Due to the high mechanical properties in these exemplary nuts, many engineers have never read the limitations on pages 3 in NASM21042 for these exemplary thin walled nut fasteners.
Additionally, some nuts fail because of conflicting hardness values (HRC) permitted in a variety of specifications. When there are conflicting permitted HRC values, hydrogen embrittlement may result in failure. A variety of alerts and revisions from the European Aviation Safety Agency (EASA), the Department of Defense (DOD), and the Federal Aviation Administration (FAA) indicate NASM21042 and NAS1291 nut failures are due to hydrogen embrittlement caused from high hardness HRC values from heat treatment.
The specifications in NASM21042 and NAS1291 allow for a maximum hardness for nut fasteners of 49 HRC. However, one example of conflicting HRC value is depicted in the Aerospace Material Specification AMS-QQ-P-416. This specifications indicates that when plating cadmium via electrodeposited processes onto fasteners that parts heat treated to an ultimate tensile strength greater than 200 ksi (or 43 HRC), should not be plated. However, each NASM21042 and NAS1291 indicates that the maximum hardness can be 49 HRC. This conflict causes some fasteners to be heat plated at a hardness value greater than 43 HRC, such as 49 HRC as permitted by NASM21042 and NAS1291. This results in hydrogen embrittlement causing failure in the fastener when it cracks.